We are using an SCR switch to apply a voltage to a high current transformer for circuit breaker testing. The two SCRs are connected antiparallel, and the gates are turned on at about 70 degrees phase angle, and then left on until the desired length AC pulse has occurred, or the breaker on the output trips and stops current flow.
The applied voltage is selected from transformer taps from 0 to 600 VAC. Sometimes when switching higher taps, the SCRs self-initiate, causing a very high output current that trips the breaker instantly, but unexpectedly. We were able to reduce this effect with a 30 uF cap and 1 ohm snubber across the tap switch, but the problem still occurred. The manufacturer had mounted a 575 VRMS MOV varistor across the SCRs, which are rated 1800 V. When we disconnected the MOV, the problem disappeared.
My question is to determine if there could be a danger to the SCRs. I have found an MOV rated 1800 V, so that might provide some protection. The maximum peak voltage that should be imposed on the SCRs is about 850 volts, and the MOV supplied breaks down at 850 to 900 volts. The snubber across the SCRs is 0.033 uF and 50 ohms. The capacitance of the MOV is probably about 0.1 uF.
I think the initial surge through the MOV is magnetizing the transformer, and then there may be inductive spikes during the contact bouncing of the switch. I think these spikes caused the MOV to conduct, resulting in a partial cycle of current. Without the MOV, the SCRs probably do not see a spike high enough to cause them to trigger on. My understanding of SCRs is that a voltage above their breakdown voltage will cause spurious triggering, and possible degradation of blocking voltage.
My other concern is about voltage spikes that may occur when the circuit breaker on the output opens up. I would expect an inductive kick, which will initially cause arcing on the breaker contacts, dissipating most of the stored energy. Since the SCRs are still turned on, I think any other inductive energy will be dissipated in the snubber on the tap switch, and the wiring of the primary circuit.
We are still having some problems when the firing circuit board is connected to the SCRs, however. I will be looking into that Monday. I think the current in the tap switch snubber is being picked up by the board, and somehow turning on the gates. The board is mounted on the SCR heat sink, so maybe it needs to be moved away or put into a shielded enclosure.
Thanks for any ideas or thoughts.
Paul